ASHINGTON, Dec. 31  The American authorities in the last week directed
a United States-bound flight from Mexico to turn around in midair and imposed
extraordinary security measures on at least six other incoming flights because
of terrorist concerns, federal officials said Wednesday.

Officials were so concerned about possible attacks on at least five foreign
flights that landed in the United States, including one on Wednesday night at
Dulles International Airport outside Washington, that they moved the planes
away from the main terminals and rescreened the passengers.

The security moves, along with the cancellation of several flights on Dec.
24 by Air France to Los Angeles, reflect an aggressive new approach toward guarding
United States airspace because of concerns that terrorists may seek to hijack
an international flight. The strategy is an outgrowth of the "high risk"
alert status initiated 11 days ago.

While officials said they wanted to cooperate with other nations to strengthen
security, they also said they were unwilling to let foreign flights into United
States airspace without rigorous security checks.

This week, Tom Ridge, secretary of the Department of Homeland Security, put
foreign airlines on notice that they would be denied entry to American airspace
if they refused to put armed air marshals on any incoming flights of concern.
The move, which met resistance from some countries, came less than a week after
Air France canceled six flights because of American worries that terrorists
might be aboard. The federal officials said that putting pressure on foreign
carriers to use marshals was just one of several steps they were taking behind
the scenes to meet rising concern about international flights.

The officials said that in the last week they also had "significantly
increased" inspection of air cargo on foreign flights, a source of widespread
concern as a potential mode of attack for terrorists.

Military F-16 fighter jets have also shadowed some foreign flights from Air
France and other airlines arriving at major American cities, including Los Angeles,
an administration official said.

And the federal officials disclosed Wednesday that after the Air France cancellations,
they reached an agreement this week with a French delegation to impose tougher
security on flights that American officials suspect may be at risk.

A key provision, the officials said, was that the French agreed to give to
United States officials passenger lists for any flights thought to be suspicious
at least one hour before take-off, rather than waiting until the flight is in
the air as is now normally done. American officials said they wanted to use
that agreement as a model for ensuring tighter aviation security in other nations
as well.

"What we're trying to do is establish protocols to be able to vet these
passenger lists before the flight takes off, and that's in the interests of
both parties," said an American official involved in the agreement. "No
one wants to be told when a flight is halfway across the Atlantic that it has
to turn around."

But American officials have shown a willingness to do just that in recent days
if questions arise about a flight's security.

In the case of a foreign carrier's flight this week from Mexico, a Transportation
Security Administration inspector based there told officials in the United States
that passengers boarding a flight for the United States had not been properly
screened, government officials said.

As one official related the exchanged that followed, American transportation
officials told the airline, "You said there were procedures in place for
that flight that weren't there. Turn it around."

The airline agreed to return the plane to Mexico and rescreen the passengers,
and the American authorities allowed the flight to complete its scheduled route,
the official said. Officials refused to disclose the city of origin or the itinerary
for the flight, citing both diplomatic and national security concerns.

Since the Bush administration put the United States on high terrorist alert
on Dec. 21, some flights have made it to the United States even amid concerns
about their security, officials said.

At Dulles airport, the police surrounded a British Airways flight from London
that landed at 7:05 p.m. Wednesday and directed it to a remote site, a security
official involved in the operation said.

Intelligence developed by American officials indicated that the route of the
flight might be a target of terrorists, and at least one name on the passenger
list appeared to match a name on a terror watch list, the security official
said.

Nothing suspicious turned up in a screening of luggage on the plane, but some
passengers were searched and interviewed late Wednesday night, and officials
said it was unclear whether the flight represented a threat.

"We're out here trying to deter and disrupt attacks," the official
said, "and that's not always immediately going to produce a guy in handcuffs."

In five or six flights coming to the United States from England, Mexico and
elsewhere, officials said, there were concerns about lapses in security in the
city of origin, intelligence about possible terrorist activity, and sometimes
both. Officials said several suspect flights landed at the Los Angeles International
Airport and another at Dulles, but they declined to provide details on the routes.

In each case, officials said, security officials met the planes and did "reverse
screenings" like the one in Dulles, interviewing passengers and searching
them for explosives, weapons and other contraband.

Before the alert level was raised to orange, or high, such screenings for flights
that had already landed were rare, a security official said.

"Clearly we're in a situation where this is happening much more frequently
than in earlier periods," the official said.

In another instance several days ago, a flight headed for the United States
from Latin America was grounded on the runway for several hours after United
States officials told the air carrier they were not satisfied that passengers
had been adequately screened.

David O'Connor, director of the United States operations for the International
Air Transport Association, a trade group that represents most international
carriers, said one critical challenge facing the United States in seeking to
strengthen air security was that safety standards vary so widely around the
world.

"Some airports and airlines are very secure," he said. "The
British, for example, have been concerned about terrorist attacks long before
9/11, and they screen passengers and baggage extensively. And in Germany, the
same."

But he added, "when you're talking about developing nations in Latin America
and elsewhere, many haven't until recently initiated any real screening procedures,
and that's where you have problems."